Two types of injury have been observed in the Kidney after O2 deprivation insults. One is a rapidly progressive transport- dependent fragmentation necrosis. The other is a more slowly developing "ischemia-like" lesion characterized by cell swelling. Previous results indicate that they differ with respect to the pathogenic roles of transport activity and ATP depletion and thus, it is possible that the two lesions represent distinct mechanisms of response to hypoxia. The purpose of this proposal is to study and compare the mechanisms of these lesions as they occur in the medullary thick ascending limb of the loop of Henle (mTAL). Morphological responses in the mTAL will be correlated with renal function and metabolism using the isolated perfused rat kidney as a model system. Major advances in the present approach over that of related previous work are the introduction of techniques to more precisely estimate effects on transport activity, ATP level and energy metabolism in mTAL itself. The experimental protocols are designed to address three issues. First, the mechanism of fragmentation necrosis is examined with emphasis on the metabolic effects of transport activity in hypoxia (i.e., ATP depletion and stimulation of mitochondrial activity). In contrast to the swelling injury, which directly correlates with ATP depletion, low energy stores appear less important in the genesis of the fragmentation lesion than the excess O2 demand resulting from stimulation of mitochondrial activity. Second, the relationship of the swelling and fragmentation lesions will be assessed. Two possibilities will be distinguished: 1) That they represent divergent pathways of response to hypoxia, or 2) that the swelling injury is an early precursor of fragmentation. The possibility that the two lesions share pathogenic mechanisms (specifically defective cell volume regulation and altered calcium homeostasis) will also be studied. Third, the role of acidosis in hypoxia will be evaluated since preliminary studies indicate that pH is an important determinant of the type of mTAL injury in anoxia. The results of these studies may have significant implications for the understanding of mechanisms of hypoxic cell injury. If there are in fact two distinct pathways of hypoxic injury the determinants and mechanisms of each would be fundamental to analysis of O2 deprivation situations. If on the other hand fragmentation necrosis represents a transports dependent acceleration of a uniform response to O2 deprivation then the import of the results of these studies will be in relation to the mechanism of the lethal phase of such injury.